Optical scanning equipment

ABSTRACT

Aerial photographic-equipment of the line-scanning type in which two parabolic mirrors lie on either side of a square-section rotor with four mirror faces that reflect the scanning beam on to a ridge mirror above the rotor and by which it is directed to an infrared detector cell. The signal from the IR cell is employed to modulate a light beam at one end of the rotor, which modulated beam is reflected out radially from the rotor optical systems within the rotor so as to traverse repeatedly a photographic film which is transported above the rotor in a direction parallel to its axis.

Gull

T 5o 1; D ited St:

OPTICAL SCANNING EQUIPMENT Frank Arthur Gull, 28 Oaklands Lane.Smallford. Hertfordshire, England Mar. 27, 1969 Inventor:

Filed:

Appl. No.:

Int. Cl. ..G02b 17/00 Field of Search ..350/7; l78/7.6

References Cited UNITED STATES PATENTS John .3 50/7 [451 Jan. 25, 19723,211,046 10/1965 Kennedy ..350/7 Primary Examiner-Rodney D. Bennett,Jr. Assistant Examiner Brian L. Ribando AnorneyDowell & Dowell [57]ABSTRACT Aerial photographic-equipment of the line-scanning type inwhich two parabolic mirrors lie on either side of a squarwsection rotorwith four mirror faces that reflect the scanning beam on to a ridgemirror above the rotor and by which it is directed to an infrareddetector cell. The signal from the IR cell is employed to modulate alight beam at one end of the rotor, which modulated beam is reflectedout radially from the rotor optical systems within the rotor so as totraverse repeatedly a photographic film which is transported above therotor in a direction parallel to its axis.

8 Claims, 7 Drawing Figures PATENTED JAN25I972 SHEET 10F 4 PATENTEUJAN25I972 SHEET 2 OF 4 PATENTED JANZS \972 sum am 4 OPTICAL SCANNINGEQUIPMENT This invention relates to scanning equipment for viewing orilluminating a scene over a wide angle, especially in connection withobtaining a photographic record of a terrestrial scene. A primary fieldof application of the invention lies in photographing the landscapepassing beneath an airborne or similar vehicle, particularly to obtain athermal" picture of the infrared part of the spectrum.

It is an object of the invention to provide scanning equipment for thistype of duty which is unusually compact and uses fewer components thansystems available hitherto, and for which the driving power required iskept advantageously low.

According to the present invention line scanning equipment comprises amultifacete d rotor with a reflecting surface on each facet, with twofixed focusing reflectors situated one on either side of the rotor toreflect beams from the faces thereof in convergent paths toward furthermeans arranged to deliver both reflected beams into a common detectiondevice. Preferably, for optical scanning, the rotor is square crosssection with four mirror faces. In the infrared case the detectiondevice will normally be an infrared sensitive electrical cell. All thepicture information can be detected by a sensitive surface or surfacesof a single cell if the fixed reflectors have mirror faces which areparts of a paraboloid and an optical element such as a ridge mirror isemployed to reflect into the detector cell beams approaching it oneither side from the two collector mirrors.

The arrangement gives spaces immediately above the scanning rotor(assuming the field of scan is below the scanner) for further equipmentin the system. To produce a photographic record in the infrared case thesignal output of the detector cell may be employed to modulate a sourceof white light projected as a focused recording beam for writing onordinary photographic film. The film transport unit can advantageouslybe located in the aforesaid space above the scanning rotor.

In the preferred arrangement the same rotor is employed not only toperform the line scan in the viewing optical system but also to providethe line scan for producing the photographic record. To achieve this thefilm can be arranged to travel above the rotor parallel to and along therotor axis and the rotor can include a portion housing a rotatingoptical system for projecting the modulated recording beam radially withrespect to the rotor so that it repeatedly traverse the film abovewidthwise. If the rotating optical system is a multiple arrangementprojecting the recording beam along a number, say four, of equiangularlyspaced radial paths each radial beam or light pencil will pass insuccession across the film so writing successive lines thereon as thefilm travels. By making the square section portion of the rotor hollowand situating the source of white light, e.g., a glow lamp, within thesquare section portion at or near one end an exceptionally compactassembly is achieved wherein all the recording'optics are carried in oraround the rotor.

By way of example, one embodiment of the invention will now be describedwith reference to the accompanying drawings, which show equipment to behoused in an airborne vehicle, i.e., a drone, for taking an infraredphotographic record of the terrain below. In the drawings:

FIG. I is an exploded pictorial view of the equipment,

FIG. 2 is a diagram of the scanning optics seen along the axis of therotor,

FIG. 3 is a pictorial diagram illustrating the operation of the scanningoptics,

FIG. 4 is a diagram of a film recorder seen in the same direction asthat in FIG. 2,

FIG. 5 shows the film recorder in the direction of the arrow 5 of FIG.4,

FIG. 6 is a diagram of the recorder optics seen from the same viewpointas that in FIG. 5, and

FIG. 7 shows the recorder optics on the line 77 of FIG. 6.

The equipment is designed as a self-contained unit to fit into a dronecamera compartment 10 which takes the form generally of a horizontalcylinder. The line scan device consists of a main frame 13 of lightalloy supporting two split paraboloid collector mirrors II, I2 disposedat either side of an aperture 14 cut into the lower half of the droneskin and structure. Between the two mirrors ll, 12 spins an opticallypolished square scanner I5, rotating at 7,500 rev./min. about ahorizontal fore and aft axis to scan the field of view across theterrain beneath the drone through the aperture 14.

Immediately above and attached to the scanner mounting 16 is a filmtransport holder 17 and film drive mechanism. Above this, centrallymounted on top of the main frame 13 is the infrared detector 18,electronics pack I9, and a DC motor 20 that drives the scanner rotorthrough a train of gears 21 at the forward end of the structure. Thefilm transport comprises a supply compartment 22 for an unused filmcassette at the front end and a compartment 23 for an exposed filmcassette at the rear, the film 24 (70mm) travelling substantiallyhorizontally from front to rear between the two. Quick access to theused film cassette is obtained by removing the scanner mounting I6 andfilm transport holder 17 in one piece, by loosening four quick-releasebolts 25, leaving the remainder of the line scan in situ, and opening ahatch 26 in an end plate 27 of the compartment 23.

The choice of the four-sided mirror results from the fact that thisconfiguration gives a reasonably optimum collecting area, consistentwith a modest power to drive the rotor and furthermore enables areasonably constant aperture to be presented during the effective scan.

From this arrangement, the largest collecting aperture can be obtainedfor the drone space and power available. Since power absorbed in windagelosses is approximately proportional to the fifth power of the rotordiameter and directly proportional to length it is important to obtainthe desired aperture area using a rotor which can be long and have asmall diameter, rather than vice versa. The scanner should therefore bedesigned, firstly, to occupy the maximum length of aperture available inthe drone skin and then to have the largest possible diameter within thelimit set by the power available to drive it. A system with a squaremirror can operate at atmospheric pressure, without the need for asealed window to enable a lower pressure to be used to reduce windage.

The fixed optical components collect the radiation reflected from thescanning rotor and focus it on to the infrared detector. These surfacesare all reflecting to avoid transmission losses which would occur iflenses were used. The split paraboloid collector mirrors II, 12 are usedto gather radiation from each side of the rotor 15 and-Ito focus it onto the detector 18 via a small reflecting ridge mirror 28. The splitcollectors allow the optical system to have minimum height, and providespace between and above them for the photographic recording equipment.

The paraboloid surfaces of the collector mirrors give a resolutionbetter than one milliradian about the scanner axis. These mirrors can bemade in glass from a single paraboloid and coated with a gold-reflectingsurface and silicon monoxide to provide excellent reflecting propertiesout to 14 microns wavelength. Such surfaces will not be susceptible todeterioration with time and a salt laden atmosphere, and can be cleanedby hand without fear of damage. Instead of glass, is suitablecompression moulded plastic, or a thin plastic coating on an aluminum orhoneycomb rigid substrate, may be employed giving savings in productioncost and in weight of the system.

The optical system is autocollimating with the scanner rotor removed sothat it can be simply focused without the need for an externalcollimated light source.

The varying signal produced by the infrared detector 18, due to theincoming radiation falling thereon, is used through the medium of aconverter unit to modulate white light which then writes upon thephotographic film. For this purpose, the square scanner rotor 15 ishollow with an enlargement midway along its length constituting ahousing 29 for recorder optics. Into one end of the hollow rotor thereprojects a stationary flow tube 30 the light output of which ismodulated in accordance with the signal output of the infrared detector18. This light is focused into a beam by a lens 31 and passes axiallythrough the rotor to be reflected back at the far end by a mirror 32.

The rotating housing 29 contains a central assembly of mirrors or prisms33 that reflect the return beam from the mirror 32 out radially alongfour paths spaced at 90 angles. in each of these radial paths there is alens system 34 for focusing the respective beam on to the film 24 abovethe scanner rotor. The four beams so focused traverse the width of thefilm in succession as the scanner rotates and thereby write" foursuccessive lines of the film record. To maintain correct focus, the filmis constrained by a curved guide so that, in the region of its travelwhere the lines of the record are written on it, it is curved widthwiseabout the axis of the scanner rotor. The speed of the scanner rotor isconstant while the film transport speed can be varied to suit the speedof the vehicle.

The whole of the detecting and recording assembly thus forms aparticularly compact assembly.

What is claimed is:

l. A scanning and recording system to be carried by a vehicle,comprising a scanning rotor mounted to rotate about an axis parallel tothe direction of motion of said vehicle and having a plurality ofequiangularly disposed reflecting faces each said face lying in a planeparallel to said axis of rotation, twin fixed focusing reflectorssituated one on either side of said rotor to reflect beams from thefaces thereof upward in two mutually converging paths, a detectiondevice giving a signal output that varies in accordance with beamstrength falling thereon which detection device is mounted above andspaced from said rotor, means receiving said two mutually convergingreflected beams and delivering them as a combined beam onto saiddetection device, record medium transport means, a record medium bandtransported by said transport means along a path extending above saidrotor parallel to said axis of rotation and passing between said rotorand said detection device, recording beam projection means mounted onsaid rotor to rotate therewith and projecting a recording beam radiallywith respect to said axis of rotation which recording beam passesrepeatedly widthwise across said record medium band transported abovesaid rotor as said rotor rotates, and means modulating said recordingbeam in accordance with variations in the signal output of saiddetection device.

2. A system according to claim I, wherein the scanning is opticalscanning and the detection device is an infrared sensitive cell.

3. A system according to claim 2, wherein said fixed focusing reflectorsare mirrors with their reflecting faces forming parts of a paraboloid,and said means receiving said two mutually converging beams is a ridgemirror oriented to reflect onto said detection device beams approachingit on either side from said two fixed paraboloid mirrors.

4. A system according to claim I, wherein said rotor is generally ofsquare cross section with four equiangularly disposed mirror faces.

5. A system according to claim 1, wherein said recording beam projectionmeans comprises multiple optical focusing systems projecting saidrecording beam along a plurality of equiangularly spaced radial paths,whereby plural radial beams pass in succession across said record mediumband so recording successive lines thereon as said record medium bandtravels.

6. A system according to claim 5, wherein said rotor is generallyhollow, a light source modulated in accordance with said detector devicesignal output disposed at one end of said rotor, and said recording beamprojection beams further comprises central reflecting optical meanswithin said rotor, the light from said light source passing axiallywithin the rotor and being reflected to said multiple optical systems bysaid central reflecting optical means.

7. A system according to claim 1, disposed in and aerial vehicle forscanning the terrestrial scene below and operating at ambientatmospheric pressure.

8. A system according to claim I, wherein said record medium band is aphotographic film, and a curved guide constrains said film, in theregion of its travel where it is scanned by said recording beam, to awidthwise curvature centered on said axis of rotation.

1. A scanning and recording system to be carried by a vehicle,comprising a scanning rotor mounted to rotate about an axis parallel tothe direction of motion of said vehicle and having a plurality ofequiangularly disposed reflecting faces each said face lying in a planeparallel to said axis of rotation, twin fixed focusing reflectorssituated one on either side of said rotor to reflect beams from thefaces thereof upward in two mutually converging paths, a detectiondevice giving a signal output that varies in accordance with beamstrength falling thereon which detection device is mounted above andspaced from said rotor, means receiving said two mutually convergingreflected beams and delivering them as a combined beam onto saiddetection device, record medium transport means, a record medium bandtransported by said transport means along a path extending above saidrotor parallel to said axis of rotation and passing between said rotorand said detection device, recording beam projection means mounted onsaid rotor to rotate therewith and projecting a recording beam radiallywith respect to said axis of rotation which recording beam passesrepeatedly widthwise across said record medium band transported abovesaid rotor as said rotor rotates, and means modulating said recordingbeam in accordance with variations in the signal output of saiddetection device.
 2. A system according to claim 1, wherein the scanningis optical scanning and the detection device is an infrared sensitivecell.
 3. A system according to claim 2, wherein said fixed focusingreflectors are mirrors with their reflecting faces forming parts of aparaboloid, and said means receiving said two mutually converging beamsis a ridge mirror oriented to reflect onto said detection device beamsapproaching it on either side from said two fixed paraboloid mirrors. 4.A system according to claim 1, wherein said rotor is generally of squarecross section with four equiangularly disposed mirror faces.
 5. A systemaccording to claim 1, wherein said recording beam projection meanscomprises multiple optical focusing systems projecting said recordingbeam along a plurality of equiangularly spaced radial paths, wherebyplural radial beams pass in succession across said record medium band sorecording successive lines thereon as said record medium band travels.6. A system according to claim 5, wherein said rotor is generallyhollow, a light source modulated in accordance with said detector devicesignal output disposed at one end of said rotor, and said recording beamprojection beams further comprises central reflecting optical meanswithin said rotor, the light from said light source passing axiallywithin the rotor and being reflected to said multiple optical systems bysaid central reflecting optical means.
 7. A system according to claim 1,disposed in and aerial vehicle for scanning the terrestrial scene belowand operating at ambient atmospheric pressure.
 8. A system according toclaim 1, wherein said record medium band is a photographic film, and acurved guide constrains said film, in the region of its travel where itis scanned by said recording beam, to a widthwise curvature centered onsaid axis of rotation.